1. Field of the invention:
The present invention relates to a composite ferrite material obtained by consolidating a high-crystallinity magnetic ferrite powder with glass, more particularly to a composite ferrite material which can readily be produced in desired dimensions. The present invention also relates to a method for the preparation of above-mentioned composite ferrite materials.
2. Description of the prior art:
Magnetic ferrite articles are manufactured mainly by the powder metallurgical method. In this method, magnetic ferrite powder is sintered by firing at high temperatures in the following manner.
First, ferric oxide powder, and other metal oxide powders such as nickel oxide, zinc oxide, etc., are mixed in specified proportions in accordance with the characteristics of the desired magnetic article, and subjected to pre-sintering. This pre-sintering results in a certain degree of solid phase reaction at the grain boundaries, and the generation of gas. The material so obtained is then pulverized, and granules of an appropriate size are formed by adding water-soluble resin to consolidate the said powder (this process will hereinafter be referred to as granulation). This granular material is then press-formed and the resulting powder mass is subjected to the final firing in a suitable gaseous atmosphere at a temperature higher than the aforementioned pre-sintering temperature. In this manner, a polycrystalline magnetic ferrite article possessing the desired magnetic properties and mechanical strength is obtained.
FIG. 4 shows the microstructure of such a polycrystalline magnetic ferrite mass obtained by sintering. This sintered magnetic ferrite mass is composed of an aggregate of porous sintered magnetic powder 6 possessing numerous pores 9. Other pores 8 are also present to some extent at the grain boundaries between the grains of said magnetic powder 6.
The temperature at which the pre-sintering is carried out in the aforementioned method is set in the range of 700.degree.-1000.degree. C., wherein a solid phase reaction is initiated at the interfaces of the original raw materials, i.e., ferric oxide, nickel oxide, zinc oxide, etc. The temperature of the final firing, performed in order to attain an adequate degree of sintering, is ordinarily set in the higher range of 1000.degree.-1400.degree. C. The temperature of the final firing that is employed varies according to the composition of raw materials, the condition of pre-sintering, the shapes and grain size of the powder after pre-sintering. The gaseous atmosphere used when firing varies according to the type of magnetic powder product desired, both oxidizing and non-oxidizing atmospheres being employed.
In the aforementioned methods, the powder obtained by pre-sintering is of grain diameter 2-5 .mu.m or less. In the mass formed by compression of this powder, the granules of the said powder are in mutual contact, but considerable gaps still remain between the granules. When the powder mass formed from this pre-sintered powder is heated at a temperature exceeding the pre-sintering temperature (700.degree.-1000.degree. C.), mutual diffusion of the atoms that constitute the granules occurs at the areas of contact between pre-sintered powder granules, and thus sintering proceeds. As sintering progresses, the gaps between the pre-sintered powder granules decrease. As a result, the final firing causes a further densification of the pre-sintered powder mass, ordinarily by a ratio ranging from 10 to 20% and in some cases even higher, which may cause deterioration in the dimensional precision and yield of the final sintered product. In order to obtain final sintered compacts of the desired dimensions, machine finishing processes such as cutting or grinding are necessary.
In general, in order to form sintered articles of uniform composition that does not crack when subjected to abrupt rises or falls of temperature, comparatively gradual elevation and reduction of temperature during the final firing is essential. Consequently, the final firing process ordinarily requires at least half a day, and in some cases may even last for two days.
Considerable research has already been conducted into efforts to improve these defects in ferrite sintering methods. For example, Japanese Laid-Open Patent Publication Nos. 58-135133 and 58-135606, discloses that when a mixture of pre-sintered ferrite powder and glass powder is press-formed, and the resulting mass is fired at an appropriate temperature sufficiently high as to allow sintering of the said magnetic powder, the said glass powder fuses, the magnetic ferrite powder granules are bound by the glass, and as a result the degree of contraction of the ferrite mass becomes relatively small. However, in the above-mentioned process, because the mass made of the powder mixture is fired at a temperature exceeding the temperature of the pre-sintering that is carried out to obtain the pre-sintered ferrite powder, a contraction of several percent occurs. This is due to the fact that, although most of the ferrite powder grains are separated from each other by the fused glass, a solid phase reaction may occur at the interfaces between the ferrite powder grains during the final firing operation.
In general, if sintering is performed in order to obtain the desired characteristics in the manufacture of sintered ferrite articles, then the further the sintering process progresses, the greater the proportion of shrinkage of the said article. In the aforementioned method, if the content of glass powder, is increased in order to suppress shrinkage, then the essential characteristics of the ferrite cannot be adequately manifested in the final product. Sintered ferrite articles are widely used as materials for electronic parts and devices, and therefore ferrite articles which combine high-level functional characteristics with dimensional precision are important desiderata.